Mössbauer study of the effect of rare earth substitution into montmorillonite

Novel montmorillonites were prepared by the exchange of the interlayer cations with a series of rare earth cations (La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, and Er) and characterized by XRD, XRF, SEM, chemical analysis and ⁵⁷Fe Mössbauer spectroscopy. An unexpected magnetically split component, assigned to iron being in the interlayer space, was observed in the Mössbauer spectra at 78K in some rare earth cation exchanged montmorillonite. This paper is the initial report about this observation. The transition of iron from the octahedral site to the interlayer and possible incorporation of rare earths in sites different from those which are in the interlayer space was concluded.     

Mössbauer study of sediment alteration produced by heat treatment

Pincenvent (France) cooking-place silt-like sediments have been studied by using Mössbauer spectroscopy and characterized by using X-ray diffraction (XRD). The sediments were treated at 400, 600, 800 and 1000°C under oxidizing conditions. We discuss the results and we suggest a Mössbauer thermometer to be used to recognize the temperatures and atmosphere ancient people used in their cooking-places.Fellow of CONICET, Argentina, on leave, Laboratoire d'Ethnologie Prehistorique, 44 Rue de L'Almiral Mouchez, 75014 Paris, France.     

Mössbauer study of iron removal in a montmorillonite

⁵⁷Fe Mössbauer spectroscopy in conjunction with atomic absorption spectrometry and X-ray powder differaction analyses have been used to study the iron present in a montmorillonite prior to and after different successive stages of two deferration processes. Fe³⁺ ions occupy mainly octahedrical M(2) sites in the mineral structure; no impurities of iron oxides were detected. The quite efficient deferration by HCl refluxing produced a substantial alteration of lamellar structure of montmorillonite, whilst dithionite/citrate treatment did not induce severe structural changes but had low iron removal efficiency.     

Mössbauer study of the annealing effect on low‐alloyed steels

Structural analyses of Fe-Cr and Fe-Cr-Ni low‐alloyed steels before and after annealing at 600^○C using Mössbauer spectrometry, X‐ray diffraction and scanning electron microscopy are reported. At the received state, the two steels present a fine bainitic microstructure with equal hardness. From Mössbauer analysis, two iron sites are identified as substitutional by Cr, Mo, Ni atoms and as insertional by carbon in bainitic ferrite. Both sites are locally deformed by residual stresses. The presence of small quantities of retained austenite and ε carbide has been observed. Annealing for one hour at 600^○C causes a decrease in hardness for both steels with a decrease of retained austenite. After longer time of annealing, precipitation of (Fe,Ni)₂₃C₆ occurs in the Fe-Cr-Ni steel and increases hardness.     

Mössbauer study of pH dependence of iron-intercalation in montmorillonite

⁵⁷Fe Mössbauer spectroscopy and XRD have successfully been applied to show the incorporation of Fe ion into the interlayer space of montmorillonite via treatment with FeCl ₃ in acetone. The 78K ⁵⁷Fe Mössbauer spectra of montmorillonite samples reflected magnetically split spectrum part indicating the intercalation of iron into the interlayer of montmorillonite via the treatment with FeCl ₃+acetone and washed with water until the initial pH=2.3 increased to pH=4.14. It was found that the occurrence of intercalated iron in the form of oxide-oxihydroxide in montmorillonite increases with the pH. Intercalation was confirmed by the gradual increase in the basal spacing d₀₀₁ with pH.     

The influence of pressure on the Mössbauer effect

A study was made of the pressure dependence of the intensity of the Mössbauer line, and the dependence of this quantity on the specific volume of the crystal, which has the same form for all these crystals in the first approximation, was derived for regular atomic crystals in the Grüneisen approximation. The change in intensity of the Mössbauer line due to pressure can be converted, except for a multiplication factor, to a change in intensity of this line caused by a change in temperature.     

Mössbauer study of gamma‴-iron nitride film

A single-phase γ?-FeN film with the rock-salt structure was produced by pulsed laser deposition of Fe onto an Al substrate in a N₂ atmosphere. Its Mössbauer spectra and powder X-ray diffraction patterns were measured. γ?-FeN was found to be antiferromagnetic exhibiting a hyperfine magnetic field of 30 T at a temperature of 5 K. It was found to have a Néel temperature of 220 K. A minor component with a higher hyperfine magnetic field of 49 T at 5 K was also observed. It is thought to originate from defects in γ?-FeN.     

Mössbauer study of products of the thermocatalytic treatment of kerogen-containing rocks

To understand the mechanism of highly efficient catalyst formation from precursors, the active form of an iron-containing catalyst and kerogen samples from the Bazhen formation are studied by Mössbauer spectroscopy and X-ray diffraction analysis. It is established that as a result of phase changes, the multicomponent precursors are transformed into mixed transition metal oxides. It is found that the thermocatalytic effect on pyrite-containing kerogen leads to the formation of pyrrhotite FeS_{1 ? x} and mixed oxides of iron.     

New applications of the Mössbauer effect

Results from a Mössbauer experiment to observe acoustic oscillations induced by pulsed laser excitation in MgO:⁵⁷Fe²⁺ crystal are presented. Time-domain spectra are satsifacorily described by the theory of the frequency modulation of Mössbauer radiation transmitted through a vibrating resonance medium. It is proposed that the D _4Ω/D _2Ω ratio of the fourth and second Fourier harmonics of the modulated radiation be used to measure the amplitude of nuclear oscillations.     

Mössbauer effect study of the magnetic structure in δ-FeOOH

Two samples of-FeOOH with different particle sizes have been studied in an external field of 4 T and as a function of temperature. They were found to have a ferrimagnetic structure due to an unequal occupancy of antiferromagnetically coupled octahedral ferric ions. The large surface contribution, which is characterized by a canted spin structure and by highly deformed Fe³⁺ co-ordinations, strongly influences the magnetic properties observed with Mössbauer spectroscopy.     

Mössbauer effect study of Ni bearing aluminous goethites

Five synthetic oxyhydroxides of iron with different contents of Al and an approximately fixed content of Ni have been studied using⁵⁷Fe Mössbauer spectroscopy in conjunction with X-ray diffraction, infrared spectroscopy, transmission electron microscopy and chemical analysis. It has been shown that, besides alumogoethite, some amorphous Al-bearing phase of iron exists, Ni being preferentially associated with this phase. Not only the effective magnetic field, but also the line width of the Mössbauer spectra at 77 K may be a measure of Al content in both phases. Doublet to sextet area ratio provides a measure of the relative concentration of the amorphous and crystalline phases.     

Mössbauer study of tektites

Room temperature ⁵⁷Fe Mössbauer spectroscopy has been used to investigate the structural and oxidation state of Fe in tektites from different strewn fields. Spectra have been analyzed in terms of two quadrupole splitting distributions corresponding to Fe^3?+? and Fe^2?+?. All tektites show similar distribution of quadrupole splitting. Each distribution has one peak. The Fe^2?+? sites show a narrow region of Mössbauer line shift (δ) and quadrupole splitting (ε), δ?= 1.02–1.10 mm/s and ε?= 0.85–1.00 mm/s relative to α-Fe. These values have been assigned to intermediate coordination between tetrahedral and octahedral. The Fe^3?+? sites show wider regions of hyperfine parameters: δ?= 0.25–0.45 mm/s and ε?= 0.65–0.90 mm/s. The Fe^3?+?/Fe^2?+? ratio was found to be 0.05–0.15.     

The Mössbauer effect and magnetism

The Mössbauer effect enabled the magnetic hyperfine splitting (hfs) in ferromagnetic solids to be observed directly for the first time. It was quickly extended to measurements on antiferromagnets, ferrimagnets and paramagnets, and is now well established as a probe for the study of magnetic materials. Applications have ranged from the old problem of the state of the iron atoms in ferromagnetic alloys to the new magnetic materials, e.g. amorphous magnets, spin glasses, fine particle magnets and multilayers. Some examples where the Mössbauer effect has made an important contribution are described.     

Mössbauer study of the Cu-Zn alloy system

Using the 93.3keV transition in⁶⁷Zn, the Lamb-Mössbauer factor, the electron density and the electric field gradient at the Zn nucleus have been determined for pure Zn metal, the ^–, ^–, ^–, and -phases as well as pure Cu metal.     

Mössbauer and EPR study of nitrosyl hemoglobin

Nitrosyl hemoglobin was prepared by bubbling fresh⁵⁷Fe-enriched rat hemoglobin with NO. S- and X-band EPR spectra at 77 K are typical for anS=1/2 system with an anisotropicg-tensor and exhibit hyperfine interactions of¹⁴N with the electronic spin. Mössbauer spectra at 4.2 and 100 K consist of a superposition of spectra from high- and low-spin Fe(III), deoxygenated hemoglobin and a component corresponding toS=1/2,g=2, hyperfine constantsA _xx /g _{n n} =A _yy /g _{n n} =–19.6 T,A _zz /g _{n n} =6.8 T, quadrupole splitting E _Q=1.5 mm s^–1, isomer shiftI _s=0.42 mm s^–1 and linewidth 0.4 mm s^–1. The spin-lattice relaxation rate at 100 K is <2×10⁶ s^–1.     

A Mössbauer effect study of MgFe2O4

A slowly cooled sample of the ferrimagnetic spinel MgFe₂O₄ has been studied with ⁵⁷Fe Mössbauer spectroscopy over a wide temperature range both with and without high magnetic fields. The observed temperature dependence of the A and B site hyperfine parameters is discussed. Conclusions about the spin structure, the magnetic exchange interactions and the supertransferred hyperfine fields are presented.     

Comparative Mössbauer study of the aging of Fe-C and Fe-N martensites

The clustering-ordering synergy which forms Fe₆C precipitates by aging Fe-C martensite is compared to the long-range ordering which forms Fe₁₆N₂ by aging Fe-N martensite.     

A Mössbauer spectroscopic study of the effect of molybdenum during preparation of Nd-Fe-B magnets

Elements in atomic ratios Nd₂(Fe_0.9Mo_0.1)₁₄B were melted in an induction furnace and annealed in order to examine the effect of molybdenum during the preparation of Nd-Fe-B magnets. A phase analysis has been made from ⁵⁷ Fe Mössbauer spectroscopic measurements in the temperature range of 100 to 700 K. It is found that two iron containing phases are formed, one Mo rich Fe alloy and the other Nd-Fe(Mo)-B isostructural to Nd₂Fe₁₄B. The Nd_1.1Fe₄B₄ phase usually found in Nd-Fe-B magnets is not observed in these samples. The Curie temperature, T_c, is found to be 605(5) K from the Mössbauer as well as vibrational sample magnetometer measurements on the same sample. At low temperatures, the average hyperfine field at Fe nuclei is found to show a decrease with respect to the value for Nd₂Fe₁₄B.     

Crystal-field effect on the hyperfine splittings,Mössbauer spectra and specific heat of Er3+ in pyrogermanate and diglycollate hosts

Hyperfine splittings and the Mössbauer spectra (MS) of the two isomers ¹⁶⁶Er and ¹⁶⁷Er in the pyrogermanate (or ErPG) and diglycollate (or ErDG) hosts were calculated using the previously determined values of the crystal field (CF) parameters and the CF levels. MS for the 166 isomer in both the hosts show only 5 lines, whereas 27 lines are expected for the 167 isomer. In case of ErDG, the quadrupole interaction P increased considerably between 300 and 4 K, i.e., from 1.5 to 21.3 MHz in case of ¹⁶⁶Er and from −1.0 to −14.0 MHz in ¹⁶⁷Er. However for ErPG, the corresponding changes in P for both the isomers are insignificant ∼1 MHz. The hyperfine magnetic fields H_hf in ErDG and ErPG were found to be nearly the same being equal to 4.4±0.01 MG because the g_∥-values are close, i.e., 10.31 and 10.58, respectively. Specific heat C_p of ErPG was measured between 30 and 0.65 K and a sharp λ peak was detected with a transition temperature T_N=0.95±0.01 K. The calculated hyperfine and Schottky specific heat components, C_hf, and C_sch, respectively showed peaks at 32 mK and 110 K. From the experimental results of C_p, the lattice specific heat C_L/R in ErPG was found to be 3.34×10⁻⁵ T³ and the thermal characteristics of the magnetic specific heat C_M was determined. The internal energy U_M(T) was obtained from C_M by numerical integration and its change ΔE/R=[U_M(∞)/R–U_M(0)/R] was found to be 1.02 K and the corresponding value of the Weiss constant θ in the ordered state was 2.01 K. The value of the exchange interaction constant |J_ex/k| and the dipole–dipole interaction constant for nearest neighbours were found to be ∼0.5 and ∼1.03 K, respectively. The molecular field (H_mol) for ErPG was equal to 4.1 kG. From future epr and MS measurements, interesting results are expected regarding the magnetic ordering in ErPG and the phase change in ErDG.     

Mössbauer study of natural wolframites

Natural Wolframite, (Fe _x Mn_1?x )WO₄ withx=0.95 to 0.41, obtained from seven different sites of two quartz-wolframites deposits of Degana and Sirohi in Rajasthan. India, have been studied by Mössbauer spectroscopy down to 20 K. X-ray diffraction studies with a monochromatic Cu radiation (λK_a-1.5405 Å), were carried out to determine the value ofx. The Mössbauer spectra of all seven samples were recored at 300, 200, 100, 50, 40, 30 and 20 K, and were least square fitted for different sites. The Mössbauer parameters are attributed to a high spin ferrous ion in a quite distorted octahedral symmetry, and only one sextet has been resolved below transition temperature.